"Masked" philadelphia chromosome (Ph1) due to an unusual translocation

The case of a 58-year-old white female with Philadelphia (Ph1)-positive chronic myelocytic leukemia (CML) and a "masked" Ph1 chromosome is described. The Ph1 was due to a translocation involving chromosomes #12 and #22, in which most of the long arm of #12 had been translocated to the deleted #22 (Ph1). The necessity of performing banding analysis in those cases of CML where a Ph1 is not readily apparent is stressed and the literature on"masked" Ph1 chromosomes is briefly summarized.